In the air separation industry or in the field of the electronic industries, many different types of high purity gases must be analyzed for quality or process control. To achieve this, there are several types of analytical instruments available on the market using different types of detectors.
Some of these detectors are specific to an impurity to be measured and their levels of susceptibility to other impurities present in the sample can be acceptable since they do not affect dramatically the accuracy of the measurement. An example of such a detector known in the art is the O2 fuel cell detector used to measure O2 impurities in gas samples. The presence of H2, N2, H2O or CH4 in about the same level of the O2 to be measured does not interfere with the O2 identification and quantification.
The measurement of Hydrocarbons by a standard configuration flame ionization detector (FID) is another example of this. The FID will not be really affected by other traces of impurities in presence in the sample like O2, N2, CO, CO2 or moisture. Thus, in such detector configurations no special consideration has to be taken to protect them against such undesirable interference.
Typical online analytical systems rely on detector that uses the chemical or physical properties of the impurity to be measured to generate an electrical signal. However, when the impurity to be measured is inert at ambient condition, such detector could not be used. A good example of this is the measurement of traces of N2 in Argon or Helium. The inertness of N2 makes it difficult to be measured at low levels. Researchers have chosen the spectroscopic emission method in the early age of the industrial analytical equipments to measure it. This method is generally known as plasma emission detection and is used with sample background gases that are easy to ionize with the help of electrical or electro magnetic field generator. An early spectroscopic emission system for the N2 in Argon measurement is described in “Spectroscopic analysis of gas mixtures”, O. P. Bochkova and E. Y A. Shreyder, Leningrad State University Academic Press, 1965 SF-4101. This concept, introduced more than 50 years ago, is still in use today.
The basic concept of this system is to get a discharge through the gas to create plasma, i.e. a gaseous ionized zone. The molecules of the background gas get excited and excite the gas molecules of N2 impurities which in turn emit various spectral emission wavelengths characteristic of N2. A common application of this concept is the measurement of N2 in Argon or Helium sample. Such technology used in this system is known as emission spectroscopy. The molecular species in the vapour phase is excited to emit light, then, the spectrum and the intensity of the emitted light are analyzed to determine the concentration of impurity in the sample.
The excitation of an atom or a molecule to a level that can emit photons requires energy greater than or equal to the excitation energy of a given level. This energy can be supplied by conversion of the kinetic energy of electrons (electrons temperature), ions or atoms colliding, absorption of light quanta and collisions of second kind. However, the later refers to radiation-less transfer of excitation energy from other particles.
In the art, almost all possible ways to create plasma and to excite the sample background (Ar or He) have been experimented for the determination of N2. The excitation ranges from direct DC or AC current discharge through the gas, the simplest form of excitation, to micro-wave induced discharge or plasma, passing by the low frequency silent electric discharge and VHF-RF frequency range. The operating pressures are also variable, ranging from low pressure, i.e. rough vacuum operating emission cell, to the atmospheric pressure. Depending on excitation mode, some discharges are of a glow type, like in low pressure, and others are of a silent electric discharge type, also called dielectric barrier discharge (DBD) or streamer discharge. This type of discharge occurs at atmospheric pressure. The power coupling technique used in the above spectroscopic emission systems, also differs. The direct coupling method is referred to a configuration where the metal electrodes are in contact with the gas to be ionized.
Such systems are described in U.S. Pat. Nos. 3,032,654; 5,412,467; 4,148,612 and 5,168,323, and in the following papers: “Spectroscopic analysis of gas mixtures”, O. P. Bochkova and E. Y A. Shreyder, Leningrad State University Academic Press, 1965 SF-4101: “Excitation de l'azote, oxygène et hydrogène en impuretés dans des décharges de gaz rares He, Ne et argon”, A. Ricard, J. Lefebvre, Revue de physique appliquée, 10 mai 1975 tome 10; “Detector for Trace Amount of Nitrogen in Helium”, R. J. Walker, Cryogenics, 1986, vol. 26 May; “A DC Microplasma on a chip employed as an optical emission detector for gas chromatography”, Jan C. T. Eijkel et Al., Analytical Chemistry vol. 72, No. 11 Jun. 1, 2000; “An Optical Emission Study on Expanding Low Temperature Cascade Arc Plasmas”, Q. S. Yu and H. K. Yasuda, Plasma chemistry and plasma processing Vol. 18, No. 4, 1998, and “Emission Spectrometric Method and Analyzer for Trace of Nitrogen in Argon”, Homer Fay, Paul H. Mohr, and Serard A Cook, Linde Co. Division of Union Carbide, Analytical Chemistry, Vol. 34, No. 10 Sep. 1962.
In all of these abovementioned references, it can be seen that for the N2 measurement, the emission wavelength typically used for low level measurement is 337.1 nm. However, 391 nm is sometimes used when sample background is Helium because of its strong intensity compared to other N2 emission lines or bands. This leads to the fact that in all the commercially available systems known today using emission spectroscopy to measure N2 in Ar, spectral emission at 337.1 nm is used to identify and quantify N2. When background is Helium, emission at 391 nm is preferred.
The wavelength of interest is filtered out by means of interference filters, grating or monochromator and its intensity is transformed into an electrical signal by any suitable photo electrical device. The electrical signal is then processed to provide the final N2 level.
Most of these systems use a single emission cell with a single optical measurement channel. It is a general belief that measuring N2 emission of an excited Argon or Helium plasma will give a linear signal from sub ppm to a few hundred ppm and no special means is generally used to correct any nonlinearity.
However, even if the above-described method is generally accepted by people involved in the art for N2 measurement, this method could become unreliable under certain conditions that are found in almost all industrial field applications. Indeed, the ideal situation would be to have a dry binary mixture of N2 in Ar or He. In such case, the measurement will not be affected by any other impurities. But even in such conditions, non-linearity occurs, as it will be described and demonstrated thereinafter, thereby resulting in substantial measurement errors.
As another example, in air separation industries, the N2 level in Argon products should be measured. Based on a particular Ar production process, many other impurities could also be present in the Ar and sometimes at a higher level than N2. Typical impurities generally found are: H2, O2, N2, CH4, CO and H2O. Furthermore, at the truck loading station, when the Ar is transferred into the tanker to be shipped to the customer, ambient air and moisture contamination is a to frequent problem. There is also the fact that, at the present time, customers of such produced high purity Argon are demanding more rigorous quality control. As consequence, customers are also asking for the measurement of other impurities like CO2 and NMHC (Non Methane Hydrocarbons), leading to believe that these impurities could also be present as an impurity in the high purity Argon.
In these situations, the presence of other impurities will have an effect on the system's performance. N2 and Ar are inert gases and normally do not react with other molecules at ambient conditions. However, inside a plasma emission system, gases under ionization state (or plasma) are a very reactive and aggressive medium. Many call it the forth state of matter, as described in “Gaze lonizate (ionized Gases)”, M. M. Badareu and Popescu, Dunod Edition. It has been known for a long time that in an ionized gas or plasma, many chemical reactions can occur even with inert gases. Plasma contains molecule radicals and atoms but also ions and free electrons which result from the coupling of energy with matters in the gaseous state. This application field relates to plasma chemistry and plasma processing.
In this field, many research works have been conducted with plasma systems similar to the ones used in analytical systems today used. Most of them use DBD or Dielectric Barrier Discharge (also called silent dielectric discharge) because of its simplicity. Thus, it is relatively inexpensive to manufacture compared to systems relying on other modes of excitation. RF excited discharge or pulse and direct current generating discharge or plasma could also be used. In this field of application, the “reactor” property of plasma is used to trigger various chemical reactions. People involved in this art have documented and reported many species, free radical and their reaction rates. They use emission spectroscopy to diagnostic or investigate plasma reactor operations.
Since most of plasma reactors used is very similar, not to say the same, as the emission cell used in the commercially available analytical instruments, they should trigger different types of chemical reactions if there is different types of impurities flowing in it. Despite this fundamental evidence it seems that, in the analytical instruments used today, no special considerations are taken to overcome interference problems.
As mentioned above, even if the previously described systems perform relatively well in the case of a dry binary mixture, except for the system linearity, it can be seen from the above that, in the real word, many other impurities could be present in the gas to be analysed, thereby leading to inaccurate measurement of the impurities to be measured. Moreover, as also mentioned above, the methods presently used today could become unreliable under particular conditions surrounding the emission system and additional impurities that could be present in the gas under analysis.
It would therefore be desirable to provide an improved method and an improved system for measuring impurities such as N2 in rare and noble gases that would be very accurate and reliable, whatever the surrounding conditions and the additional impurities that could be present in the gas under analyse. Moreover, it would be even more desirable to provide such a method that would provide accurate measurements even at sub-ppb and up to 10,000 ppm levels while providing a long-term stability.